1. Field of the Invention:
The invention relates generally to recording and reproducing scrambled television signals and, more specifically, relates to apparatus for storing descrambling codes to use in descrambling the recorded signals for subsequent playback in an unscrambled mode.
2. Description of the Prior Art:
Subscription television systems are well known in which a broadcasting station transmits a scrambled or coded television signal to the subscriber who has been provided with a decoder after making the appropriate financial arrangements with the broadcasting company. The decoder unscrambles or decodes the transmitted signals for viewing by the subscriber and, thus, unless the appropriate decoder is provided the video signals cannot be properly displayed. In most cases, the audio signals are also scrambled.
One proposed method for descrambling the signal in such a subscription television broadcasting system periodically inverts the polarity of the video signal for a predetermined period of time. Another example of a descrambling system periodically changes the order or sequence of the signal for a predetermined period of time. The corresponding code to correct the polarity or sequence of the signals is then contained in the decoder provided by the broadcasting company, so that the signals can be descrambled.
There are various other methods that can be used to scramble the signal, and in one kind a key code is broadcast along with the scrambled signal. This key code is based on the code used to scramble the signal originally and, thus, the subscriber can descramble the signal to obtain the appropriate audio and video signals.
Another system has been proposed in which a transmitted television signal is received and recorded directly in scrambled form by a video tape recorder (VTR) and, subsequently, the recorded and scrambled signal is played back through a decoder for viewing. Another feature of this proposed system, is that the scrambled signals are broadcast during off-peak hours, that is, at times other than the usual broadcast hours for prime-time programming, for example, after midnight.
One problem which has arisen in transmitting scrambled program signals and then descrambling them at the decoder is caused when distortion is introduced into the signal, either over the transmission path or upon receiving the signals at the receiver. The principal problem relates to the fact that during the step of reinverting the signals in polarity to decode them, if some distortion has been introduced there will be signal level differences at each of the polarity inversions and attendant waveform distortions will result. This produces a disconcerting visual flicker when the descrambled signals are displayed on a video monitor. The flicker is present at each point where the signal has been reinverted in polarity, and the flicker is particularly conspicuous when viewing still pictures, that is, when viewing one video frame. Moreover, such flicker is even more likely to be produced when the scrambled video signal has been recorded and played back by a video tape recorder, because such video tape recorders typically employ nonlinear DC circuits and nonlinear AC circuits, which emphasize the level differences and accentuate the flicker.
Another feature of the system of transmitting and recording scrambled broadcast signals at times other than prime viewing time is the ability to prevent viewing of the program on a real-time basis. This can be accomplished by providing the key signal, which is necessary for descrambling, at or near the end of the program. Therefore, the program must be completely received, or at least received in part, before it can be decoded for viewing, and this involves the memorization of the key code in a memory for subsequent read out into a decoder to descramble the signal.
Not only is the prevention of viewing programs in real-time a desirable feature but also the limitation on the number of times that a recorded program may be viewed is also desirable. That is, assuming that the scrambled signal has been recorded by the video tape recorder along with an appropriate key code, which has been stored in a suitable memory, the program then can be replayed an indefinite number of times, however, it is advantageous to those owning the rights to the program material to limit the number of times that a subscriber can decode the scrambled signal and, thus, the ability to limit the number of times that the program can be played back is an advantage.